Abstract

This document collects together into one definition all the specs that
together form the current state of Cascading Style Sheets (CSS) as of
2007. The primary audience is CSS implementors, not CSS authors, as this
definition includes modules by specification stability, not Web browser
adoption rate.

Status of this document

This section describes the status of this document at the time of
its publication. Other documents may supersede this document. A list of
current W3C publications and the latest revision of this technical report
can be found in the W3C technical reports
index at http://www.w3.org/TR/.

Publication as a Working Draft does not imply endorsement by the W3C
Membership. This is a draft document and may be updated, replaced or
obsoleted by other documents at any time. It is inappropriate to cite this
document as other than work in progress.

The (archived) public
mailing list www-style@w3.org (see
instructions) is preferred
for discussion of this specification. When sending e-mail, please put the
text “css-beijing” in the subject, preferably like this:
“[css-beijing] …summary of
comment…”

This specification is a Last Call Working Draft. All
persons are encouraged to review this document and send comments
to the www-style
mailing list as described above. The deadline for
comments is 15 August 2010.

1. Introduction

When the first CSS specification was published, all of CSS was contained
in one document that defined CSS Level 1. CSS Level 2 was defined also by
a single, multi-chapter document. However for CSS beyond Level 2, the CSS
Working Group chose to adopt a modular approach, where each module defines
a part of CSS, rather than to define a single monolithic specification.
This breaks the specification into more manageable chunks and allows more
immediate, incremental improvement to CSS.

Since different CSS modules are at different levels of stability, the
CSS Working Group has chosen to publish this profile to define the current
scope and state of Cascading Style Sheets as of late 2007. This profile
includes only specifications that we consider stable and for
which we have enough implementation experience that we are sure of that
stability.

Note that this is not intended to be a CSS Desktop Browser Profile:
inclusion in this profile is based on feature stability only and not on
expected use or Web browser adoption. This profile defines CSS in its most
complete form.

Note also that although we don't anticipate significant changes to the
specifications that form this snapshot, their inclusion does are not mean
they are frozen. The Working Group will continue to address problems as
they are found in these specs. Implementers should monitor www-style and/or
the CSS Working Group Blog for
any resulting changes, corrections, or clarifications.

1.1. The W3C Process and CSS

This section is non-normative.

In the W3C
Process, a Recommendation-track document passes through five levels of
stability, summarized below:

Working Draft (WD)

Published during the process of drafting the specification, the
purpose of a public Working Draft is to create a snapshot of the
specification's current state and to solicit input from the W3C and the
public. The document is known to be unstable, and is often incomplete.

Last Call Working Draft (LC or LCWD)

By publishing a Last Call Working Draft, a working group is expressing
that they consider the spec to be complete and all issues to be resolved.
Publishing a Last Call Working Draft announces that this specification
will move toward Candidate Recommendation unless significant issues are
brought up. The Last Call period is a last chance for others to submit
issues before the transition to CR.

Candidate Recommendation (CR)

By publishing a Candidate Recommendation, a working group is
expressing that have resolved all known issues and they believe the spec
is ready for implementation.

Proposed Recommendation (PR)

To exit CR and enter this stage, the spec needs a comprehensive test
suite and implementation reports proving that every feature in the spec
is interoperably implemented in at least two shipping implementations.
Entering the Proposed Recommendation stage signals to the W3C that these
requirements have been met. Once the W3C officially approves the
specification, it becomes a Recommendation.

Recommendation (REC)

This is the final stage. At this point there should need to be no more
changes.

In the CSSWG's experience, the recommendation track is not linear. The
wider review triggered by an LCWD often results in at least another
working draft, possibly several. More significantly, our experience is
that many specs enter CR twice, because implementation testing often
uncovers significant problems in the spec and thus pushes it back to
working draft. Additionally, fixing even minor problems forces a CR to
re-enter the Working Draft stage. As a result, although the CSSWG has a
clear idea of the stability of the CSS specs, it is very difficult for
someone outside the working group to come to that same understanding based
on a specification's official status. The CSS Working Group's motivation
for creating this document is thus to communicate to others our
understanding of the state of CSS.

2. CSS Levels

Cascading Style Sheets does not have versions in the traditional sense;
instead it has levels. Each level of CSS builds on
the previous, refining definitions and adding features. The feature set of
each higher level is a superset of any lower level, and the behavior
allowed for a given feature in a higher level is a subset of that allowed
in the lower levels. A user agent conforming to a higher level of CSS is
thus also conformant to all lower levels.

2.1. CSS Level 1

The CSS Working Group considers the CSS1 specification
to be obsolete. CSS Level 1 is defined as all
the features defined in the CSS1 specification (properties, values,
at-rules, etc), but using the syntax and definitions in the CSS2.1 specification. CSS Styling Attributes Level
1 defines its inclusion in element-specific style attributes.

2.2. CSS Level 2

Although the CSS2
specification is technically a W3C Recommendation, it passed into the
Recommendation stage before the W3C had defined the Candidate
Recommendation stage. Over time implementation experience and further
review has brought to light many problems in the CSS2 specification, so
instead of expanding an already unwieldy
errata list, the CSS Working Group chose to define CSS Level 2
Revision 1 (CSS2.1).

CSS2.1 is now a Candidate Recommendation—effectively though not
officially the same level of stability as CSS2—and should be
considered to obsolete the CSS2 Recommendation. In case of any conflict
between the two specs CSS2.1 contains the definitive definition. Features
in CSS2 that were dropped from CSS2.1 should be considered to be at the
Candidate Recommendation stage, but note that many of these have been or
will be pulled into a CSS Level 3 working draft, in which case that
specification will, once it reaches CR, obsolete the definitions in CSS2.

2.3. CSS Level 3

This section is non-normative.

CSS Level 3 builds on CSS Level 2 module by module, using the CSS2.1
specification as its core. Each module adds functionality and/or replaces
part of the CSS2.1 specification. The CSS Working Group intends that the
new CSS modules will not contradict the CSS2.1 specification: only that
they will add functionality and refine definitions. As each module is
completed, it will be plugged in to the existing system of CSS2.1 plus
previously-completed modules.

From this level on modules are levelled independently: for example
Selectors Level 4 may well be defined before CSS Line Module Level 3.

3. Cascading Style Sheets Definition

As of 2007, Cascading Style Sheets
(CSS) is defined by the following specifications. Each specification
in this list builds on and possibly modifies the definitions in the
previous specifications, with the base formed by CSS Level 2
Revision 1. (In other words, CSS is defined as CSS Level 2
Revision 1, modified by CSS Namespaces, modified by
Selectors Level 3, etc.) A valid CSS document is one that
conforms to this definition.

3.1. Partial Implementations

So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS layout implementations must
treat as invalid (and ignore as
appropriate) any at-rules, properties, property values, keywords, and
other syntactic constructs for which they have no usable level of support.
In particular, user agents must not selectively ignore
unsupported property values and honor supported values in a single
multi-value property declaration: if any value is considered invalid (as
unsupported values must be), CSS requires that the entire declaration be
ignored.

3.2. CSS Profiles

Not all implementations will implement all functionality defined in CSS.
For example, an implementation may choose to implement only the
functionality required by a CSS Profile. Profiles define a subset of CSS
considered fundamental for a specific class of CSS implementations. The
W3C CSS Working Group defines the following CSS profiles:

3.3. Experimental
Implementations

To avoid clashes with future CSS features, the CSS2.1 specification
reserves a prefixed
syntax for proprietary property and value extensions to CSS. The CSS
Working Group recommends that experimental implementations of features in
CSS Working Drafts also use vendor-prefixed property or value names. This
avoids any incompatibilities with future changes in the draft. Once a
specification reaches the Candidate Recommendation stage, implementors
should implement the non-prefixed syntax for any feature they consider to
be correctly implemented according to spec.